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Exciting Ideas In Solar Energy From ARPA-E

Now in its fourth year, the summit of the Advanced Research Projects Agency-Energy (ARPA-E) never fails to bring out the most cutting-edge ideas in renewable energy. This week’s conference in Washington D.C. is no exception. I walked the exhibition floor today and ran across some sexy new concepts in solar power.

Solar Vortex: Dust Devil Power

The Solar Vortex borrows its inspiration from dust devils, those miniature twisters of excited dirt that sometimes arise in the dusty and dry stretches of the U.S. Southwest. What gets a dust devil going is the difference in temperature between the scorching-hot ground and the somewhat cooler air above. The hot air rises, twists and gives rise to a momentary dust tornado.

GeorgiaTech is the leader of a consortium that aims to capture this dust-devil energy inside a stubby cylinder. The concept is simple: The cylinder sits upon a dark surface that absorbs lots of heat. The “walls,” so to speak, are angled vanes that take the hot air rising off that hot surface and twist it into a vortex. At the top, a set of fan blades sit in the path of the rising air. The fan blades turn, activating a generator that creates electricity.

The video below is a miniature model of the Solar Vortex on the exhibition floor. The cylinder sits on a plate that is, like hot pavement, almost too hot to touch, about 47 degrees Celsius (116 degrees Fahrenheit). The movement you see in the blade is solely from the force of moving air.

Georgia Tech has already purchased gotten rights to use a site in Mesa, Arizona — plenty of heat there — and is working toward building a 50-kilowatt commercial-scale model. Final negotiations with ARPA-E are underway for an intermediate step: a 10-kilowatt version by 2015 . Arne Pearlstein, a professor of mechanical engineering who is a collaborator, told me that the commercial-scale version might be 10 meters wide but only two or three meters tall, and that the units would sit about 55 meters apart. These squat machines could bring renewable energy to regions that are bombarded by heat but don’t have much wind. (Though gusts of wind would only serve to make the turbine spin faster, Pearlstein said.)

Pearlstein estimated that the Solar Vortex could spin out electricity 20 percent cheaper than wind turbines and 65 percent cheaper than solar photovoltaic panels. One form of saving comes from its potentially straightforward maintenance. “You’re talking about somebody getting up on a stepladder instead of going hundreds of meters up into a wind turbine to deal with a gearbox,” Pearlstein said.

A model of Otherlab's heliostat, which goes by the name Sunfolding.

Sunfolding: Cheap Heliostats for Concentrated Solar

Two weeks ago, San Francisco’s Otherlab won a $1.8 million grant from ARPA-E to develop Sunfolding, a smaller, less expensive heliostat for solar concentrating technology. A heliostat is the mirror and assembly that bounces light onto a central point, creating heat that in turn creates steam and spins a turbine to generate electricity. Making heliostats cheaper could bring us closer to the day when concentrated solar energy is comparable in price to fossil fuels like coal or natural gas.

Heliostats are usually made of glass (or another reflective material) and steel, and are often quite sizable. They must stand up to high winds while maintaining a precise orientation. Simultaneously, in order to follow the arc of the sun across the sky, they need gears that can operate for years in dusty and hot conditions. Both are factors that increase costs.

The centerpiece of the Sunfolding is a new tracking technology that does away with a gearbox and replaces it with two air bladders that inflate or deflate to change the mirror’s angle. “All you need are two cheap pressure sensors,” said Leila Madrone, Otherlab’s project leader. The housing for those bladders is a section of extruded plastic that could be manufactured easily. The smaller size means it needn’t stand up to high winds or much structural weight.

A Raven drone decked out with MicroLink solar panels.

MicroLink Devices: Light, Flexible Solar for Drones

Lying on MicroLink Devices’ table at the ARPA-E conference is a Raven UAV, a small drone used by the U.S. military for local video reconnaissance in places like Afghanistan. The topside of the wings are covered with solar panels that, according to Vice President David McCallum, have in tests extended the Raven’s flight time from one hour to two.

Drones are a promising market for solar power, since solar panels don’t provide nearly the wattage necessary to keep a passenger plane aloft, but can do the trick when a craft (like the Raven) weighs only four pounds.

Lightweight, flexible solar panels have been available for a while now in the form of thin-film solar panels, but MicroLink says it can produce loads more electricity than thin films. Thin-film solar has demonstrated a maximum efficiency of 20.4 percent, while MicroLink says it can convert sunlight to electricity at an efficiency of 30 to 34 percent.

MicroLink’s strong suit is that it has figured out how to make triple-junction solar cells — ones that use multiple layers to capture different wavelengths of light — more cheaply than the competition. It does this in two ways, by growing its solar panels on indium phosphide, which is cheaper than the more standard material of gallium arsenide. The company has also figured out how to peel its completed solar panel off while leaving its substrate intact for future use, which again reduces costs.

In December, the company received a $3.3 million grant from ARPA-E with the goal of creating a concentrated-solar panel that has an efficiency of 50 percent or more.

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